1. Field of the Invention
The present invention concerns a magnetic resonance apparatus of the type having a gradient coil with passive shim devices inserted into receptacles in the unit, each shim device being formed by a number of ferromagnetic metal elements.
2. Description of the Prior Art
To improve the homogeneity of the basic magnetic field within the measurement volume of a cylindrical magnetic resonance apparatus, shim devices (formed by disc-shaped, laminar metal elements, in particular iron discs) are used that are inserted into separate receptacles in the form of pockets molded into the gradient coil unit and arranged around the measurement volume in a cylindrical wall. This metal or iron arrangement is known as a passive shim. The shim arrangement in total can have several hundred metal discs. The locations of the discs and their number are normally determined via an iterative calculation method for which the measurement field homogeneity is measured relative to a spherical volume and the required iron quantity is calculated for the sites of the respective receptacles on the gradient coil unit. When the required iron quantity per shim device is known, the very thin iron plate discs are counted by hand and inserted into the apparatus compartments of the carrier. Such a carrier is executed as a type of drawer or tray that has a number (for example 16) of rectangular compartments molded back-to-back and open at the top, into which the individual discs (which sometimes vary with regard to their size and thickness in order to be able to approach as optimally as possible the calculated iron quantity) are inserted. Each compartment at the carrier is then manually sealed with a cover plate and the shim device is inserted into the coil receptacle. This method is very time-consuming and occupies approximately 2-3 days for a new installation. Errors are easily possible since, as stated, the discs must be counted and inserted by hand. It is easily possible to count too few or too many discs or to insert discs of the wrong size etc. A further disadvantage is that the occupancy of a pocket can be checked only by removing the entire disc packet, since the discs are not visible from the exterior of the pocket due to being arranged lying atop one another.